The present invention relates generally to improving the properties of intermetallic compositions. More particularly, it relates to improving the ductility of iron-aluminides.
It is well-known that aluminide intermetallics have received much attention recently in research programs because of their potential as high temperature materials for structural applications. In general, aluminides of various metals are found to have relatively low density because of the presence of the aluminum. They also have good temperature stability at elevated temperatures and in addition relatively poor ductility at lower temperatures. Iron-aluminide, FeAl, is one of the group of aluminide intermetallics. Iron-aluminide is one of the high temperature aluminides which do not fail elastically at room temperature under tension loading. However, fracture of these aluminides does occur intergranularly in conventional equiaxed high aluminide alloys.
I have found that directional solidification of iron-aluminide can generate an elongated grain structure. Since the area of grain boundaries traverse to a stress direction is minimized for directionally solidified iron-aluminides, a directionally solidified structure is deemed to be resistant to intergranular failure such as those noted above to occur in conventional equiaxed high aluminide alloys. Ideally, single crystal intermetallics can completely eliminate the grain boundary and can consequently eliminate the grain boundary brittleness problem. Accordingly, high temperature properties can be significantly improved by the employment of directionally solidified or single crystal iron-aluminide materials. Similar phenomena have been demonstrated in the case of cast superalloys.
Directional solidification changes the mode of failure for iron-aluminides from an intergranular mode to a transgranular mode at low temperatures Ductile Brittle Transition Temperature (DBTT) for such directionally solidified compositions decreases to about 300.degree. C. The tensile elongation for such directionally solidified iron-aluminide compositions at room temperature has an upper limit of about 2-3%.
What is sought pursuant to the present invention is improvement in the lower temperature tensile elongation of directionally solidified iron-aluminides.